unlikely has much to do with your opener so much as always using mindblast when you can, and always having an eye out for enemy healthbars (on adds fights always have healthbars displayed) to snipe them with shadow word death. generating extra orbs means extra devouring plagues (and insanity if you're using that talent).

Your spell power is very low. Reforge neck,ring and trinket to get more haste, and replace some haste gems by intellect ones.

Opener:
MB
SW:P
VT
Halo

This will do more dmg for u as dot's have a chance to proc trinkets, and u have 2 additional tick's of sw while only delaying halo by few secs... Everything except mind flay is worth casting before Halo.

My Rule of Thumb:1. Shadow Orb Generation is top priority: Always cast MB on CD and snipe with SWD (just the first SWD, wait 6 secs before recasting again. No need to spam it twice in a row UNLESS your next spell is [Non-SWI] Mind Flay, then cast SWD again)2. Casting DP when you can, but make sure you cast it before your next cast of MB/SWD Snipe. (I tend to wait on my DP, hoping for a trinket proc)3. If you talented into SWI, Multi-dotting on adds yield more DMG (VT, SWP, and DP) then casting DP + Mind Flay.4. Reapply DoTs with weapon/trinket procs + Lust/Heroism and any mechanic DMG increase (ie- Jin'rokh the Breaker Water Pool Buff). It's easier to reapply SWP then casting VT.

Other Tips:1. Always position yourself where you don't have to move much.2. When talented with ToF, always try to snipe low HP for the buff uptime.3. When casting Insanity, interrupt Mind Flay with ~1s left on your DP to cast another empowered MF (and get another few ticks out of the Insanity).

When I was first learning to play shadow, I started using the addon “Spellflash” to help with the rotation. Now while I’m awaiting incoming flames about how spellflash is not going to provide optimal DPS, I can assure you that it will certainly get you very close. I basically turns what can be a tricky DPS rotation into a hit-the-button-that-lights-up whack-a-mole rotation. This made the class rofleasy to learn for me. If you decide to give it a try, you will have to download three separate addons: Spellflash, Bitten’s Spellflash Library, and Bitten’s Spellflash: Priest.